[wilf leek]

MSD 6AL outputs multiple sparks up to around 3000 rpm (i.e. 2 or more sparks per firing point). After that, one spark per TDC. (or whatever your timing is).

Could be this is upsetting your timing light. I believe that inductive timing lights are specifically warned about by MSD - could be worth a check on their site/call to their tech line.